Carbon Dioxide Exchange in Lichens: Apparent Photorespiration and Possible Role of CO2Refixation in Some Members of the Stictaceae (Lichenes)

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Abstract
At low O2 concentration (1% O2, v/v) the true photosynthetic (TP) rates of Pseudocyphellaria homoeophylla and Sticta latifrons are stimulated by up to 46% and 55% respectively. This stimulation, which is considered to be due to the repression of photorespiration (PR), varied with the thallus water content of the lichens. At intermediate and low water contents PR rates approximated the 43% of TP theoretically expected for C3 plants, but at very high water contents PR rates were near zero. Results for P. billardierii show that in this species PR rates are low (up to 11% TP) at all water contents. Comparison of light/dark CO2 evolution ratios under low O2 conditions indicated that up to 70% of respired CO2 could be refixed within the lichen thallus. The lack of stimulation at 1% O2 and high water content is suggested to be a result of a high surface CO2 diffusion resistance encouraging internal CO2 refixation. Lichens thus join the group of plants with low CO2 compensation values and photorespiration.

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